Waterbed mattress bags are generally used with a circumscribing frame so that containment and lateral support of the mass of water is sustained by a relatively sturdy structure rather than by the sheet film material of the waterbed mattress bag itself. By using a peripheral support structure, it is possible to relieve the upper layer of the mattress bag from stress whereby a person resting upon the mattress bag is supported largely by buoyancy effects and only minimally by tension of the sheet film material. Popularization of this "flotation" principle is generally attributed to Charles Hall. See, for example, his U.S. Pat. No. 3,585,356.
The top layer of the waterbed mattress bag must provide some support in order to stabilize the body of the person during rest or sleep. Furthermore, users prefer various degrees of firmness or flotation. Firmness depends upon the total load imposed upon the mattress bag. Thus, for example, if one lightweight person occupies a relatively large bed, the flotation effect is greater than if, say, two persons occupy the same bed. The flotation effect decreases and tension imposed by the top bag layer is increased.
It has been proposed to provide means for automatically adjusting the fill. See, for example, my U.S. Pat. No. 3,999,235, entitled PRESSURE EQUALIZER FOR WATERBED.
Notwithstanding precise instructions, users often overfill waterbed mattress bags in an attempt to achieve what they believe to be proper adjustment. Actually, good flotation requires that unloaded, the top layer of the bag be slightly slack. Unfortunately, the sheet film material does not have the elasticity necessary to maintain a regulated firmness for widely varying loads and widely varying degrees of fill.
Accordingly, the primary object of the present invention is to provide a structure whereby the firmness of the mattress is maintained at a substantially constant level, notwithstanding variations in the degree of fill and variations in the load imposed upon the mattress.
Another object of the present invention is to provide a firmness regulated waterbed mattress which not only achieves the foregoing object, but which, in addition, provides a "motionless" or damping characteristic by an interior baffle structure. The combination of firmness control and dampened motion achieves a desirable sensation of support and comfort.